This invention relates to a method of manufacturing a a color cathode ray tube shadow mask from an iron-nickel alloy.
In a conventional color cathode ray tube (CRT) as shown in FIG. 1, three electron beams 1, 2, and 3 from separate electron guns (not shown) are correctly radiated onto red, green and blue phosphors 7, 8, and 9 coated on the inner surface of a panel 6. The beams strike the phosphors after passing through apertures 5 perforated in a shadow mask 4. The phosphors 7, 8, and 9 then emit red, green and blue light to form a color image.
A shadow mask in a color CRT of this type must satisfy certain specific requirements. Small apertures must be correctly formed in a regular pattern. The shadow mask must be curved with a predetermined radius of curvature. The distance (to be referred to as the q value hereinafter) between the shadow mask and the inner surface of the panel must be maintained at a predetermined value.
When the color CRT is operated, the beam current passing through the apertures in the shadow mask comprises about one-third or less of the total beam current originally emitted by the electron guns. The remaining electrons bombard the shadow mask, which is, in some cases, heated to a temperature of 353 K. As a result, the shadow mask thermally expands to have a q value different from the predetermined q value, thus causing the "dome phenomenon." When the dome phenomenon occurs, the color purity of the CRT is degraded. The material conventionally used for a shadow mask, and which contains nearly 100% iron, such as aluminum-killed decarbonized steel, has a coefficient of thermal expansion of about 12.times.10.sup.-6 /K at 273 K. to 373 K. This material is thus easily vulnerable to the dome phenomenon.
In view of this problem, Japanese Patent Publication No. 42-25446, Japanese Patent Disclosure No. 50-58977 and Japanese Patent Disclosure No. 50-68650 propose the use of an iron-nickel alloy, which has a small thermal expansion coefficient, as the material of a shadow mask. However, this proposal has not yet led to practical use of such a material in a shadow mask. One of the reasons preventing the practical use of such a material is the difficulty of working a metal sheet consisting of an iron-nickel alloy. In order that the q value fall within a predetermined allowable range, the curved surface of the shadow mask should be controlled with high precision. For example, the allowable error for a radius of curvature R of 1,000 mm is as small as .+-.5 mm.
An iron-nickel alloy has an extremely high elasticity and a high tensile strength after annealing, as compared to ordinary iron. Accordingly, the iron-nickel alloy tends to a greater degree to return to its orignal shape when one attempts to deform it into a curved surface by pressing it in a mold. When a 200-um-thick sheet of any material is pressed into a shadow mask in the mold of FIG. 8, the resulting mask is considered acceptable if its maximum deformation d from the designed surface at a predetermined position on the shadow mask is 20 um or less after the mask is removed from the mold. Deformation d is illustrated in FIG. 2, an exaggerated view.
FIG. 3 shows the measured relationship between deformation d and yield strength for a 14 inch shadow mask. Yield strength is the tension at which the length of the material increases by 0.2%, sometimes called "0.2% proof strength." From this curve it can be seen that in order to maintain the deviation at or below 20 um, yield strength must not be greater than 19.6.times.10.sup.7 N/m.sup.2. (Since iron-nickel alloys do not clearly show the yielding phenomenon, throughout the specification tensile strength is substituted for 0.2% proof strength for these alloys.)
FIG. 4 compares the yield strength of conventional aluminum-killed decarbonized steel, curve (a), with that of an iron-nickel alloy, curve (b), for various annealing temperatures. Both curves are for shadow masks annealed in hydrogen in an annealing furnace generally used for the conventional aluminum-killed decarbonized steel shadow mask. As can be seen from FIG. 4, even if the iron-nickel shadow mask is annealed at the relatively high temperature of 1173 K., the yield strength still drops to only about 28.4.times.10.sup.7 -29.4.times.10.sup.7 N/m.sup.2.
As explained above, since shadow masks made of an iron-nickel alloy have a small thermal expansion coefficient, their use substantially eliminates degradation in color purity due to thermal deformation of the mask. However, degradation in color purity due to inability to form the mask to the proper shape (d less than or equal to 20 um) still remains.